Tennis courts and other sports use well-marked boundaries to define whether a ball is in play or out of play. If any part of a ball contacts the white line, it is in play; if it is outside of the white line it is out of play. In tennis, the boundaries are definitive for both serves and for rallies in accordance with rules of the International Tennis Federation.
In competitive tennis, calls are usually made by line judges who view from an extrapolation of the line being judged, and calls may be over-ruled by an umpire sitting on a chair along the net line at the center of the tennis court. Balls may be traveling at more than 100 miles per hour, and very fast observation and perception is required for accurate calls.
Two widely used types of tennis courts are clay courts and hard courts. Clay courts are made of crushed shale, stone or brick, and can be either red or green. The red clay is slower than the green, or Har-Tru® “American” clay. On clay courts, a mark is left on the surface. In case of a disputed call, the umpire may make a close-up inspection to discern the exact location of the ball impact.
Hard courts are usually made of cement or asphalt. Hard courts use a painted finish; generally, no impact marks are left on the court surface that can be used in the event of a line call dispute.
Although line judges and/or umpires use their best ability to make accurate calls, sometimes errors happen and could happen. Many calls are pivotal in the outcome of professional tennis matches. There is need for a simple, efficient means of improving the accuracy of line calls.
The Cyclops®, Mac Cam®, Hawk Eye®, ShotSpot® and Auto-Ref® systems have been tried. Cyclops® (William Carlton, Malta) uses a periscope system operated by two people per line, and it gives a personal line of sight very close to the boundary surface. The Cyclops® evolved into a system using five parallel infrared beams that could be interrupted by the ball in motion, and it gives colored light response if in play and an audible response if out. This system has been used only for service line calls.
The Mac Cam® (DEL Imaging Systems, LLC., Cheshire, Conn.) system uses high-speed cameras, which can be reviewed in case of a controversial line call. This system has only been used as theatre for the T.V. audience, and has not been developed for use in international tournaments.
Signal Processing Systems, Inc. (Sudbury, Mass.) has developed a system using wires imbedded in a ribbon 0.2 mm thick that is placed within the boundary, perhaps over-coated with the paint of the court. Ball hits on the ribbon sends an auditory signal to earphones being worn by the line umpires, but steps by players give more of a crunch sound.
The Hawk Eye® (Winchester, England) system uses up to nine high-speed digital cameras with computer software to track and map the impact point of the ball. Each camera is connected by a digital video cable to a computer; an impact point is displayed on the video screen at the umpire's chair or on a large T.V screen within the stadium for public viewing.
The philosophy of tennis calls presently leans in the direction of having live individuals make the calls, with opportunity for disputing a call immediately, as is done on the few clay courts presently used for professional tennis matches.
The International Tennis Federation (ITF)(London, England) has published a detailed list of requirements for automated line-calling systems (“Automated Line-Calling Systems: ITF Evaluation, ITF Technical Centre (February 2005)). In short, any new system must improve considerably upon the 40-mm (1.6 inch) best accuracy of human line umpires and it must not affect normal play in any manner. An instant call is desired, audible to umpire, players and audience.
The International Tennis Federation has recently approved an electronic/optic system for ball tracking that gives a historical record of ball location and impact, which provides a means for reviewing disputed calls. This is an elaborate and expensive system for installation and operation.
However, the majority of tennis courts cannot use many of these sophisticated and expensive systems. There is a need for a reliable and relatively inexpensive system for determining the impact position of tennis balls on a tennis court.